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Screensize: 640 x 480 800 x 600 1024 x 768 1280 x 1024 UserName: Password: Anti-Spam Code: Format Mode: Basic  Help  Prompt  Format: Font Andale Mono Arial Arial Black Book Antiqua Century Gothic Comic Sans MS Courier New Georgia Impact Lucida Console Script MT Bold Stencil Tahoma Times New Roman Trebuchet MS Verdana   Size 1 2 3 4 5 6   Color Black Red Yellow Pink Green Orange Purple Blue Beige Brown Teal Navy Maroon LimeGreen Message: * HTML is OFF * Forum Code is ON Smilies [quote][i]Originally posted by DrGonzo[/i] [br]Origin Ver. and Service Release (Select Help-->About Origin): OriginPro 8G SR2 v8.0891 (B891) I got a problem with the nonlinear curve fitting. I use the formular: dependent variable y independent variable x fit parameters a, b y=log(a*(0.254829592*1/(1+0.32759*x/b)-0.284495736*(1/(1+0.32759*x/b))^2+1.421413741*(1/(1+0.32759*x/b))^3-1.453152027*(1/(1+0.32759*x/b))^4+1.061405429*(1/(1+0.32759*x/b))^5)*exp(-x/b*x/b)) which is a erfc function. The fitting plot realy fits (r^2>0.99) my data but the strange thing is that the calculated variables are wrong in comparison to the values easyplot, igor and different data analyser calculate (I fitted the data several times with a few analyser with different guesses for a,b and just origin calculate different values). The origin values for "a" are around 20 and should be 400-600. The values for "b" are around 2200 and should be 1400. I really do not understand how this can happen. I hope somebody can help me. If necessary i could send the data and the fdf file. I would really like to use origin as my analyser. Thanks a lot [/quote]   Check here to include your profile signature. Check here to subscribe to this topic.